1. Field of the Invention
This invention relates to a toner used in electrophotography, electrostatic recording or the like. More particularly, it relates to a magnetic toner with insulating properties, an image forming method making use of such a magnetic toner, and a process cartridge detachable from the body of an image forming apparatus having the magnetic toner.
2. Related Background Art
A number of methods are hitherto known for electrophotography, as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications No. 42-23910 (U.S. Pat. No. 3,666,363) and No. 43-24748 (U.S. Pat. No. 4,071,361) and so forth. In general, copies or prints are obtained by forming an electrical latent images on a photosensitive member by utilizing a photoconductive material and by various means, subsequently developing the latent images by the use of a toner to form visible images (toner images), and transferring the toner images to a transfer medium such as paper if necessary, followed by fixing by the action of heat or pressure or both of them.
Various developing methods by which electrostatic latent images are formed into visible images by the use of a toner are also known. For example, they include a number of developing methods such as the magnetic brush development as disclosed in U.S. Pat. No. 2,874,063, the cascade development as disclosed in U.S. Pat. No. 2,618,552, the powder cloud development as disclosed in U.S. Pat. No. 2,221,776, the fur brush development and the liquid development.
In these developing methods, the magnetic brush development, the cascade development and the liquid development, which employ two-component developers mainly composed of a toner and a carrier, are particularly put into practical use. These methods are all superior methods which can relatively stably given good images, but on the other hand they have common disadvantages involved in the two-component developer, which are such that the carrier may deteriorate and the mixing ratio between the toner and the carrier may vary.
To eliminate such disadvantages, developing methods employing one-component developers comprised of a toner only are proposed in variety. In particular, many superior methods are seen in methods employing developers comprising toner particles having magnetic properties.
Various methods or devices have been developed in relation to the step of fixing toner images to a sheet such as paper, which is a final step in the above electrophotographic process. A method most commonly available at present is a pressure heat system using a heat roller.
The pressure heat system using a heat roller is a method of carrying out fixing by causing a toner image side of an image-receiving sheet to pass the surface of a heat roller whose surface has releasability to toner while the former is brought into contact with the latter under pressure.
Since in this method the surface of the heat roller comes into contact with the toner image of the image-receiving sheet under pressure, a very good thermal efficiency can be achieved when the toner image is fixed onto the image-receiving sheet, so that the fixing can be carried out rapidly. Thus, this method is very effective in a high-speed electrophotographic copying machines.
Especially in the future, copying machines will be designed for higher-speed copying, hence toners to be used should be improved in their fixing performance on recording mediums such as paper, and satisfy good image density and high operational performance (good durability) in high-speed development.
In such a heat roller fixing method, polyolefin wax is conventionally added in toner so that its anti-offset properties can be improved.
Since polyolefin wax, however, does not have good compatibility with binder resin in toner, faulty dispersion of polyolefin wax may occur when the toner is produced, causing free polyolefin at the time of pulverization.
The faulty dispersion of polyolefin wax in the toner results in not only faulty cleaning and deterioration of anti-offset properties during operation of a copying machine, but also an increase in non-uniformity of toner chargeability to cause a decrease in image density during the operation.
Japanese Patent Application Laid-open Nos. 50-81342, 56-144436, 58-11953 and 60-184260 disclose toners employing a fatty acid ester or a wax having an ester component.
In the technique disclosed in these, the ester component is not a fatty acid ester not having a long-chain alkyl group. Hence, when applied in high-speed development carried out at a process speed of 380 mm/sec or higher, improvement in fixing performance and anti-offset properties can not be said to be satisfactory. Especially when applied in a toner having an average particle diameter smaller than 10 .mu.m, the faulty dispersion of wax in binder resin may occur to cause non-uniform toner chargeability due to charge-up in an environment of low humidity, so that image density may be reduced during operation.
Especially in the future, the particle diameter of toners will be made smaller, and hence the dispersibility of wax components is sought to be more improved.
As the particle diameter of toner becomes smaller, the charge-up may come into question especially in an environment of low humidity, which is accompanied by an unavoidable problem of decrease in image density.
EP-A-0606873 discloses a toner containing as a binder resin a polyester resin at least part of which has been modified with a compound having i) a long-chain alkyl group having 22 to 102 carbon atoms and ii) a hydroxyl group or carboxyl group at its terminal. This compound, however, is obtained by reaction on a resin which has such a large molecular weight as the polyester resin, and therefore, EP-A-0606873 is directed to an invention having a concept different from the present invention.